The present invention relates to an improved method and apparatus for separating chrominance and luminance information in a quadrature modulated color television signal and similar periodically recurring signals. More particularly, the method and apparatus of the present invention effectively applies adaptive comb filtering techniques as well as vertical transition detection to a quadrature modulated subcarrier color television signal in order to provide improved separation of chrominance and luminance signal components during aperiodic transition states.
In consideration of designs for color television systems and signal formats, it has been recognized that the human eye does not require as much detailed chrominance (color hue and saturation) information as is required for luminance (brightness) information for the same apparent picture sharpness or resolution. Since the eye is extremely sensitive to brightness changes insofar as resolution is concerned, most television system signal formats, such as the National Television System Committee (NTSC) format, devote approximately 4 MHz of signal bandwidth to luminance information (the NTSC standard being 4.2 MHz). For color hue or saturation changes which are not accompanied by corresponding changes in luminance, the eye is satisfied with one-third to one-tenth the resolution needed for brightness. Thus, encoded color sub-carrier signals are accorded much less bandwidth than the bandwidth devoted to the luminance information. In the NTSC system, the I color subcarrier component has a 1.3 MHz bandwidth while the Q color subcarrier component occupies only 500 KHz. A consequence of bandwidth limitations upon color information subcarrier signals is that high frequency transitions, i.e., minute picture details, are monochrome, and that the color information applies only to larger picture details.
To reduce visible dot interference in the television picture, the NTSC color subcarrier frequency of 3.579545 MHz was chosen so that its sidebands are at odd multiples at half of the line scanning frequency. The result is that the color sidebands are interleaved between the high frequency sidebands of the luminance signals. Thus, successive dots or picture elements resulting directly from the subcarrier on one scanning line are offset and interleaved with dots on the next scanning line. The interleaved pattern of dots resulting from the subcarrier has heretofore been generally accepted by most viewers of black and white receivers as a noticeable, but usually unobjectionable, grid pattern within the picture.
Some of the reasons why the color subcarrier dot pattern have not been objected to or even perceived by the viewer are that the subcarrier is at a high frequency, so that the dots appear very close together and are not therefore objectionably visible at the normal viewing distance from the picture screen. As already noted, the dots are interleaved from one scanning line to the next, which further reduces their visibility at the normal viewing distance. Furthermore, most monochrome television receivers of domestic United States origin are characterized by a relatively low picture bandwidth and therefore will not even reproduce the dot pattern on the screen. Finally, since most information in a television picture is generally of low saturation and since it is rare to observe a fully saturated color image outside of a color bar test pattern, for example, the intensity of the dot pattern from the color subcarrier is not much greater than the ambient luminance levels upon which the subcarrier dot pattern rides. It has been found that the average color saturation of a television picture is around 20%. Thus, the brightness of a subcarrier dot rarely exceeds 20% of the ambient luminance present on the scanning line.
With pictures displayed on color television receivers, the interleaving of chrominance sidebands between luminance sidebands at the upper end of the luminance channel spectrum works well for static and low saturation color picture information, so long as chrominance levels do not exceed 20%. When high chrominance energy levels are present, the dot pattern levels in the luminance path are excessive, and combined with the high gamma of the picture tube, are shifting upwards the average brightness of saturated areas. As a result, for example, a high saturation red will be displayed on the color screen as a pinkish color. It is therefore mandatory, in a color television display, to filter out the subcarrier from the luminance channel prior to application to the display device.
The heretofore commonly employed expedient in color television receivers has been to install a band-reject filter (usually an LC trap) in the luminance path with a low end 3 db attenuation point being between 2.3 and 2.8 MHz. As a result, an incoming transition with 150 nanoseconds rise time will be degraded to 250 to 300 nanoseconds. Thus, it is appreciated that heretofore there has been a significant and continual loss of picture resolution in most color television receivers as a tradeoff for improved color purity. Also, the band-reject filter introduced ringing on trailing edges of high speed transitions and pulses such as those exemplified by the sine-square test pulse. Such ringing added additional visible artifacts to the picture which departed from the true image.
Another problem encountered when imperfect chrominance-luminance separation techniques were used resulted from the demodulation of high frequency luminance components, mistakenly detected as chrominance information by the television set demodulator. As a result fast luminance transitions were often followed by a rainbow pattern when a simple bandpass filter was used to remove luminance components from the chrominance channel.
Comb filters have been known to separate interleaved components in a complex spectrum with minimum degradations.
As early as 1930 in U.S. Pat. No. 1,769,920, F. Gray described the utilization of a comb filter design to enable interleaving two distinct television picture signals into a single spectrum corresponding to the audio frequency range.
With the advent of the NTSC color television signal format, in which the color subcarrier is interleaved between the high frequency sideband components of the luminance signal, it was proposed to employ the Gray filtering technique to the separation of chrominance and luminance signal information in a color television receiver. Gray's technique was noted in U.S. Pat. No. 3,542,945 to N.W. Parker, issued Nov. 24, 1970. Parker implemented a form of comb filter by combining a bandpass filtered composite video signal (chroma with interspersed luminance components) in an adder with a second input to the adder being bandpassed video first having been passed through a one horizontal scanning line delay line and a phase inverter. Since two successive lines of NTSC chroma subcarrier are 180.degree. out of phase by reference to horizontal synchronization pulses, the two inputs combined as a color subcarrier sum. Since lines of luminance are originally in phase, the combination of the oppositely phased luminance component inputs to the adder resulted in cancellation of luminance. Thus, a comb filtered chrominance output was provided in which luminance components had been phase cancelled. Parker's circuit then added the separated chrominance without further phase reversal to the composite video input signal (luminance plus chrominance). Phase cancellation of chrominance then occurred which provided a separated luminance output.
In static pictures having low chrominance levels, the Parker comb filter technique worked wall. However, for sudden color changes along a vertical axis and other dynamic changes in chrominance signals within the television picture, Parker's comb filter did not eliminate subcarrier dot patterns and other highly visible aberrations and artifacts deviating from the true picture on account of momentary phase shift and consequent loss of interleaving of the color signal within the upper frequency portion of the luminance signal. Another weakness of the Parker approach was the introduction of ringing and echoes in luminance transitions due to the imperfections of 1-line delay lines. Those errors were visible even when the comb filter was not required, that is, when the picture color saturation was low.
The usefulness of comb filtering in specialized television applications such as in time base error correction within color video recording and playback systems was disclosed in two of the present applicant's prior U.S. Pat. Nos. 3,674,920 issued July 4, 1972, and 3,764,739 issued Oct. 9, 1973.
In an article entitled "Comb Filter Improvements with Spurious Chroma Deletion," published in SMPTE Journal Vol. 86, No. 1, Jan. 1977, pages 1-5, Arthur Kaiser proposed a comb filter color decoder in which the comb filter was replaced during chroma transitions by a conventional low pass filter in the form of a trap. Kaiser's "adaptive" comb filter, while somewhat effective for its intended purpose, was not particularly effective during diagonal chroma transitions; and, implementation of Kaiser's circuitry as blocked out in FIG. 9 of his article, required two full bandwidth delay lines (4.2 MHz bandwidth modules which were then, and still are, very expensive). As a result, Kaiser's circuitry was practically limited to those situations, such as in television studios, where demanding video requirements may have justified the high cost of implementing his chrominance and luminance separation scheme. Another defect, known as "scalloping," was due to the presence of light vertical lines during chroma horizontal or oblical transitions, and was created by the use of two delay lines and perfect vertical alignment of chroma combing residues from line to line. Another exposition of the Kaiser technique is found in Kaiser's U.S. Pat. No. 4,072,984, issued Feb. 7, 1978. A significant difference between the Kaiser approach and applicant's approach is found in the fact that the Kaiser techniques do not use combed chrominance directly to control the switching of the low pass filter in and out of the luminance channel. Also, Kaiser does not teach or suggest that switching may validly be controlled by low frequency luminance transitions occurring in the vertical domain.
Another complex adaptive comb filtering technique for television signals is set forth in Rossi's U.S. Pat. No. 4,050,084, issued Sept. 20, 1977. Therein, Rossi's complex system does not use combed chrominance directly to control the switching in and out of the low pass filter and the luminance signal path. Also, the complexity of the Rossi switching approach involves a concept which is not capable of being implemented at low cost. Moreover, there was no teaching or suggestion in Rossi that the switching in of the low pass filter could be based on but a two line consideration, with only three choices: those being the main line component M; one-half the main line component minus the adjacent top line component, i.e., 1/2 M - T; or, zero, the third choice.
A low cost, highly effective adaptive comb filter method and apparatus was described in applicant's co-pending U.S. patent application, Ser. No. 886,164 filed Mar. 13, 1978 entitled "Method and Apparatus for Separation of Chrominance and Luminance with Adaptive Comb Filtering in a Quadrature Modulated Color Television System," now U.S. Pat. No. 4,179,705. While the system provided a vast improvement in adaptive comb filtering techniques over the prior art approaches, since the chroma processing therein was linear addition of two successive scanning lines of chroma information, there was a slight chroma fringing in the vertical dimension which became noticeable when electronic inserts or "keys" were made into the picture. Since half of the chroma information was delayed by one line, in the event of such electronic inserts or keys, the picture carried the illusion of a slight lack of chroma registration in the vertical dimension at such transitions in the picture. Thus, in some instances where electronic inserts and keys are made in the program material on a frequent basis, a need has arisen adaptively to switch off chroma combing at the transition of the electronically inserted material in the picture. Furthermore, it has been found that vertical chroma transitions are generally accompanied by luminance vertical transitions. Low frequency luminance vertical transition therefore provides a very reliable source of information to generate switching signals in order to switch off chroma combing or band-reject luminance information during chroma vertical transitions and effectively complement the mechanism described in U.S. patent application Ser. No. 886,164 in borderline cases of imperfect combing.